Device substrate, light emitting device and driving method of light emitting device

ABSTRACT

A light emitting device comprising a light emitting element and a first transistor and a second transistor controlling current to be supplied to the light emitting element in a pixel; the first transistor is normally-on; the second transistor is normally-off; a channel length of the first transistor is longer than a channel width thereof; a channel length of the second transistor is equal to or shorter than a channel length thereof; gate electrodes of the first transistor and the second transistor are connected to each other; the first transistor and the second transistor have the same polarity; and the light emitting element, the first transistor and the second transistor are all connected in series.

TECHNICAL FIELD

The present invention relates to a light emitting device having aplurality of pixels including a means for supplying an electroniccurrent to a light emitting element and a light emitting element, andfurther to a driving method of the light emitting device.

BACKGROUND ART

Since a light emitting element is self-luminous type, it has a highlevel of visibility and does not require a backlight that is needed in aliquid crystal display device (LCD). This, it is suitably applied tothinner devices and not restricted in viewing angle. Therefore, a lightemitting device using a light emitting element has recently been drawingattentions as a substitute display device for a CRT and an LCD.

An OLED (Organic Light Emitting Diode) is one example of light emittingelements and includes a layer containing an electroluminescent materialthat can obtain electroluminescence generated by applying an electricfield thereto (referred to as an electroluminescent layer), an anodelayer and a cathode layer. The electroluminescent layer is providedbetween the anode and the cathode, and further, comprises one or aplurality of layers. These layers may contain an inorganic compound insome cases. The electroluminescence in an electroluminescent layerincludes a light emission when a singlet excited state returns to aground state (fluorescence) and a light emission when a triplet excitedstate returns to a ground state (phosphorescence).

Next, a structure of a pixel of a conventional light emitting device andthe driving method thereof will be described briefly. A pixel shown inFIG. 10 has TFTs 80 and 81, a capacitor element 82 and a light emittingelement 83. A gate of the TFT 80 is connected to a scanning line 85. Asource or a drain of the TFT 80 is connected to a signal line 84, andthe other is connected to a gate of the TFT 81. A source of the TFT 81is connected to a terminal 86. A drain of the TFT 81 is connected to ananode of the light emitting element 83. A cathode of the light emittingelement 83 is connected to a terminal 87. The capacitor element 82 isprovided for storing a voltage between the gate and the source of theTFT 81. A predetermined amount of voltages is applied from a powersource to each of the terminals 86 and 87, which have a voltagedifference from each other. The term voltage in this specification meansan electrical difference from a ground, unless especially mentioned.

A voltage of a video signal inputted to the signal line 84 is inputtedinto the gate of the TFT 81 when the TFT 80 turns ON by a voltage of thescanning line 85. A gate voltage (a voltage difference between the gateand the source) of the TFT 81 is determined according to the inputtedvoltage of the video signal. Moreover, a drain current of the TFT 81that flows according to the gate voltage is supplied to the lightemitting element 83, and the light emitting element 83 emits light bythe supplied current.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

Now, a TFT formed from polysilicon has a higher field effect mobilityand a higher ON current than a TFT formed from amorphous silicon.Therefore, the TFT formed from polysilicon is more suitable for atransistor of a light emitting device. However, after all, theelectrical characteristics are inferior to characteristics of a MOStransistor that is formed over a single crystal substrate though a TFTis formed from polysilicon. For example, the field effect mobility isequal to one-tenth or less than that of a single crystalline silicon.Moreover, the TFT formed by using polysilicon has a problem that ittends to have a variable characteristic due to a defect formed on grainboundaries.

In the pixel shown in FIG. 10, there is a problem that the amount of thedrain current of the TFT 81 is different in each pixel when a thresholdvoltage of the TFT81 varies in each pixel even though a voltage of avideo signal is same, and consequently, the luminance of the lightemitting element 83 varies.

Moreover, a decrease in a luminance of a light emitting element by adeterioration of an electroluminescent material is a serious problem inputting a light emitting device to practical use. The luminancedecreases when electroluminescent materials deteriorate even if acurrent that is supplied to a light emitting element is constant. Thus,there is a problem that differences of a deterioration of a lightemitting element are generated in each pixel and that the luminance hasa variation in the case where a gradation of each pixel is differentcorresponding to a displayed image, since the degree of thedeterioration depends on time of luminescence or the quantity of theflowing current.

In view of the above-described problems, it is an object of the presentinvention to provide a light emitting device, a driving method of thelight emitting element, and a device substrate which can suppress avariation of a luminance of a light emitting element in each pixel dueto a difference in characteristics of TFT, and which can suppress adecrease in luminance of a light emitting element or a generation of anunevenness in luminance accompanying a deterioration of anelectroluminescent material.

Means for Solving the Problems

In the present invention, in addition to a transistor that suppliescurrent to a light emitting element (a driving transistor), a transistorthat functions as a switching element (a current control transistor)connects in series to the driving transistor. Both of the drivingtransistor and the current control transistor have the same polarity,and further, the gate electrodes of them are connected to each other.Moreover, in the present invention, a ratio of L/W of a channel length Lto a channel width W of the driving transistor is allowed to be higherthan L/W of the current control transistor. In addition, the drivingtransistor operates in the saturated region and the current controltransistor is allowed to operate in the linear region. Specifically, inthe driving transistor, L is allowed to be higher than W, and morepreferably, L/W is allowed to be 5/1 or more. In addition, in thecurrent control transistor, L is allowed to be equal or shorter than W.

Moreover, in this specification, a light emitting element indicates anelement whose luminance is controlled by current or voltage, andincludes an OLED (Organic Light Emitting Diode) and a MIM type electronsource element (an electron emission element) and the like which is usedfor an FED (Field Emission Display).

A light emitting device includes a panel in which a light emittingelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. Moreover, the present inventionrelates to a device substrate that corresponds to one of the modesbefore a light emitting element is completed in the steps ofmanufacturing the light emitting device, and the device substrate isprovided with a means for supplying current into a light emittingelement in each of the plural pixels.

Moreover, in the present invention, a threshold voltage Vth of thedriving transistor is set to be higher than that of the current controltransistor in the case where each polarity of the driving transistor andthe current control transistor is p-type. Conversely, in the case whereeach polarity of the driving transistor and the current controltransistor is n-type, a threshold voltage Vth of the driving transistoris set to be lower than that of the current control transistor. Acontrol of the threshold voltage can be performed by adjusting dose, orthe like, of an impurity that gives a conductive type to a channelforming region. The current control transistor is allowed to beinvariably normally-off. The driving transistor may be normally-off, butnormally-on is more preferable.

FIG. 1A is a circuit diagram showing a part of a pixel of the presentinvention. The reference numeral 101 is a driving transistor, thereference numeral 102 is a current control transistor, and the referencenumeral 103 is a light emitting element. FIG. 1A shows a case where thedriving transistor 101 and the current control transistor 102 are eachp-type, but they may also be n-type. The driving transistor 101, thecurrent control transistor 102 and the light emitting element 103 areconnected in series, and has a structure in which a drain current Id ofthe two transistors 101 and 102 is supplied to the light emittingelement 103. In addition, gate electrodes of the driving transistor 101and the current control transistor 102 are connected to each other. Anelectric potential that is given to a terminal 106 is given to the gateelectrodes of both of the driving transistor 101 and the current controltransistor 102.

Moreover, a voltage Vdd is applied between a terminal 104 that isconnected to a source (S) of the driving transistor 101 and a terminal105 that is connected to a cathode of the light emitting element 103.The driving transistor 101, the current control transistor 102, and thelight emitting element 103 are all connected in series. Therefore, thesum of a drain voltage Vds1 of the driving transistor 101, a drainvoltage Vds2 of the current control 102, and a voltage Ve1 between ananode and the cathode of the light emitting element 103 corresponds to avoltage Vdd.

In addition, in FIG. 1A, the current control transistor 102 is providedbetween the driving transistor 101 and the light emitting element 103,but the present invention is not limited to this structure. The currentcontrol transistor 102 may be connected to control a supply of the draincurrent of the driving transistor 101 to the light emitting element 103.

FIG. 1B shows the voltage-current characteristics of the drivingtransistor 101, the current control transistor 102, and the lightemitting element 103 which are shown in FIG. 1A. In addition, the graphof the voltage-current characteristics shown in FIG. 1B shows a graph110 which indicates the relation of the drain current Id to the drainvoltage Vds1 of the driving transistor 101, a graph 111 which indicatesthe relation of the drain current Id to the drain voltage Vds2 of thecurrent control transistor 102, and a graph 112 which indicates therelation of the current flowing in the light emitting element 103 to thevoltage Ve1.

Since the driving transistor 101, the current control transistor 102,and the light emitting element 103 are connected in series, the heightof a value of the current Id which flows in each element is the same.Moreover, the driver circuit 101 operates in the saturated region, andthe current control transistor 102 operates in the linear region.Therefore, the drain current Id1 at the intersection point n1 (operationpoint) of the graph 110 of the graph 112 is lower than the drain currentId2 at the intersection point n2 (operation point) of the graph 111 ofthe graph 112. Therefore, the driving transistor 101 and the lightemitting element 103 operates in the operation point n1, and the currentcontrol transistor 102 operates so that the drain current is Id1 sincethe current which flows in each element is Id1.

At this time, Ve1 is a voltage between an electric potential of thecathode and an electric potential of the operation point, and Vds1+Vds2is a voltage between an electric potential of the terminal 106 and anelectric potential of the operation point. Moreover, |Vds2| is notablysmall in comparison with |Ve1| and |Vds1| since the current controltransistor 102 operates in the linear region. Therefore, it can be saidthat Vdd≈Ve1+Vds1. In addition, in the case where the operation point n1is in the saturated region, the drain current Id1 of the drivingtransistor 101 follows the next numeral 1. In the numeral 1, β=μC₀W/L, μindicates a mobility, C₀ indicates a gate capacitance per unit area, andW/L indicates a ratio of a channel width W to a channel length L of achannel forming region.

Id1=β(Vgs−Vth)²/2  [numeral 1]

From the equation shown in the numeral 1, it can be said that thecurrent Id1 is determined only with Vgs, not Vds1. Therefore, when thevoltage Vdd is a fixed value, the value of the drain current Id is keptconstant according to the equation shown in the numeral 1, even when thevalue of Vds becomes low, instead of making the value of Ve1 high due toa deterioration of the light emitting element. Consequently, a declineof luminance can be suppressed even when the light emitting elementdeteriorates, since luminance of the light emitting element is inproportion to the current.

Incidentally, in the case where both of the driving transistor 101 andthe current control transistor 102 each operate in the linear region,Ve1 is notably high with respect to the sum of the drain voltage Vds1and Vds2. That is, it can be said that Vdd≈Ve1 because ofVe1>>Vds1+Vds2. Thus, the value of Ve1 is almost fixed even though thelight emitting element deteriorates, and therefore, a decrease ofluminance cannot be suppressed. Accordingly, an advantage that cansuppress a decrease of luminance due to a deterioration of a lightemitting element can be obtained by making the driving transistor 101operate in the saturated region, which cannot be obtained with theoperation in the linear region.

However, there is a problem that the current which is flowing in thelight emitting element is dependent on a variation of the thresholdvoltage Vth as the numeral 1 shows, since a ratio of |Vgs| to |Vth| inthe saturated region is smaller than in the linear region. In thepresent invention, the absolute value of a gate over drive voltage (thegate voltage Vgs—the threshold voltage Vth) of the current controltransistor 102 can be allowed to be higher than that of the drivingtransistor 101, even though a gate voltage in the same height is appliedby controlling the threshold voltage. More specifically, the gate overdrive voltage has a negative value in the case where the drivingtransistor 101 is p-type, and has a positive value in the case where thedriving transistor 101 is n-type. Therefore, in the saturated region,the operation point can be set in an area where a linearity of an ONcurrent to Vgs is higher, thus a variation of the ON current can besuppressed in comparison with the case of normally-off, even though thethreshold voltage, the sub threshold coefficient, the mobility or thelike varies.

Moreover, in the present invention, the variation of the ON current dueto the variation of the threshold voltage, the sub thresholdcoefficient, the mobility or the like can be more suppressed, since alinearity of the saturated region is made high by raising L/W. Inaddition, higher ON current can be obtained even though the height ofthe gate voltage is same, since the gate over drive voltage is higherthan in the case of normally-off, and a decrease of the ON current canbe compensated by raising L/W.

EFFECT OF THE INVENTION

By the above-mentioned structure, the present invention can suppress avariation of luminance of a light emitting element between each pixel,due to a difference of characteristics of a transistor. Moreover, thepresent invention can suppress a decrease in luminance of a lightemitting element accompanying a deterioration of an electroluminescentmaterial or a generation of an unevenness in luminance.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiment modes of the present invention will be describedwith reference to the drawings. However, the present invention can becarried out in various different modes, and it is easily understood bythose who are in the art that the modes and details herein disclosed canbe modified in various ways without departing from the scope and spiritof the present invention. Therefore, it should be noted that the presentinvention should not be interpreted as limiting to the presentembodiment modes.

Embodiment Mode 1

FIG. 2 shows one embodiment mode of a pixel that is included in a lightemitting device of the present invention. The pixel shown in FIG. 2includes a light emitting element 204, a transistor (a switchingtransistor) 201 which is used as a switching element to controlinputting a video signal to the pixel, and two transistors 202 and 203which control supply of current to the light emitting element. In thisembodiment mode, the transistor 202 corresponds to a driving transistor,and the transistor 203 corresponds to a current control transistor.Moreover, a capacitor element 205 to store an electric potential of thevideo signal may be provided in the pixel as the embodiment mode.

The switching transistor 201 may be n-type or p-type. The drivingtransistor 202 and the current control transistor 203 each have the samepolarity. In this embodiment mode, each of them has a p-type, but theymay have an n-type. And a threshold voltage of the driving transistor202 is set to be higher than that of the current control transistor 203.More preferably, the driving transistor 202 is set to be normally-on.Moreover, in the present invention, L/W of the driving transistor 202 isset to be higher than L/W of the current control transistor 203. Inaddition, the driving transistor 202 is allowed to operate in thesaturated region, and the current control transistor 203 is allowed tooperate in the linear region. Specifically, in the driving transistor202, L is allowed to be higher than W, and more preferably, the L/W isallowed to be 5/1 or more. In addition, in the current controltransistor 203, L is allowed to be equal to or shorter than W.

Moreover, a gate of the switching transistor 201 is connected to ascanning line Gj (j=1 to y). One of the source and the drain of theswitching transistor 201 is connected to a signal line Si (i=1 to x),and the other is connected to each gate of the driving transistor 202and the current control transistor 203. The driving transistor 202 andthe current control transistor 203 are connected in series. In addition,the driving transistor 202 and the current control transistor 203 areconnected to a power supply line Vi (i=1 to x) and the light emittingelement 204 so that a current which is supplied from the power supplyline Vi is supplied to the light emitting element 204 as a drain currentof the driving transistor 202 and of the current control transistor 203.In this embodiment, a source of the current control transistor 203 isconnected to the power supply line Vi (i=1 to x), and a drain of thedriving transistor 202 is connected to a pixel electrode of the lightemitting element 204.

The light emitting element 204 includes an anode, a cathode and anelectroluminescent layer provided between the anode and the cathode.When the anode is connected to either the driving transistor 202 or thecurrent control transistor 203, the anode is a pixel electrode and thecathode is a counter electrode. Meanwhile, when the cathode is connectedto either the driving transistor 202 or the current control transistor203, the cathode is the pixel electrode and the anode is a counterelectrode. A voltage is applied from a power supply to each of thecounter electrode of the light emitting element 204 and the power supplyline Vi so as to supply a forward bias current to the light emittingelement 204.

One of the two electrodes of the capacitor element 205 is connected tothe power supply line Vi, and the other is connected to each gate of thedriving transistor 202 and the current control transistor 203. Thecapacitor element 205 is provided to store a gate voltage of the drivingtransistor 202 and of the current control transistor 203, when theswitching transistor 201 is in an unselected state (off-state). Althougha structure in which the capacitor element 205 is provided is shown inFIG. 2, the present invention is not limited to this structure, and thecapacitor element 205 is not necessarily provided.

In the case where the source or the drain of the driving transistor 202is connected to the anode of the light emitting element 204, it isdesirable that the driving transistor 202 is a p-channel typetransistor. On the other hand, in the case where the source or the drainof the driving transistor 202 is connected to the cathode of the lightemitting element 204, it is desirable that the driving transistor 202 isan n-channel type transistor.

Next, a method of driving of the pixel shown in FIG. 2 is described. Anoperation of the pixel shown in FIG. 2 can be explained by dividing intotwo periods: a writing period and a storage period. First, when thescanning line Gj is selected in the writing period, the switchingtransistor 201 whose gate is connected to the scanning line Gj turns ON.Next, a video signal that is inputted to the signal lines S1 to Sx isinputted to each gate of the driving transistor 202 and the currentcontrol transistor 203 via the switching transistor 201. FIG. 3A showsan operation in the case where the driving transistor 202 and thecurrent control transistor 203 are ON by the video signal, and FIG. 3Bshows an operation in the case where the current control transistor 203is OFF. In addition, in FIGS. 3A to 3D, the switching transistor 201which is used as the switching element and the current controltransistor 203 each are shown as a switch.

As shown in FIG. 3A, in the case where the driving transistor 202 andthe current control transistor 203 turn ON by the video signal, acurrent is supplied to the light emitting element 204 via the powersupply line Vi. At this time, the current which flows in the lightemitting element 204 is determined by the voltage-currentcharacteristics of the driving transistor 202 which operates in thesaturated region and the light emitting element 204, since the currentcontrol transistor 203 operates in the linear region at this time. And,the light emitting element 204 emits light at a height of luminancewhich measures up to the supplied current.

Moreover, as shown in FIG. 3B, in the case where the current controltransistor 203 turns OFF by the video signal, the supply of the currentto the light emitting element is not carried out and the light emittingelement 204 does not emit light. Note that according to the presentinvention, it is possible to control not to supply the current to thelight emitting element 204 can be done, since the current controltransistor 203 is normally-off even when the driving transistor 202 isnormally-on.

In the storage period, the switching transistor 201 is turned OFF bycontrolling an electric potential of the scanning line Gj, and then anelectric potential of the video signal that is written in the writingperiod is stored. FIG. 3C shows an operation in the storage period inthe case where both of the driving transistor 202 and the currentcontrol transistor 203 are turned ON in the writing period as shown inFIG. 3A. The supply of the current to the light emitting element 204 ismaintained, since the electric potential of the video signal is storedin the capacitor element 205. Moreover, FIG. 3D shows an operation inthe storage period in the case where the current control transistor 203turns OFF in the writing period as shown in FIG. 3B. The supply of thecurrent to the light emitting element 204 is not carried out as in thewriting period, since the electric potential of the video signal isstored in the capacitor element 205.

Note that, in the pixel that is shown in this embodiment mode, the videosignal can be either a digital signal or an analog signal. In the caseof a digital signal, a gradation can be expressed by dividing one frameperiod to a plurality of periods (sub frame period), and controllinglight emission or non-light emission in each period by the video signal.Moreover, in the case of an analog signal, a gradation can be expressedby controlling an ON current of the driving transistor with the electricpotential of the video signal.

By the above-mentioned structure, a variation of luminance of a lightemitting element between each pixel can be suppressed, due to adifference of characteristics of a transistor. Moreover, a decrease inluminance of a light emitting element or a generation of an unevennessin luminance can be suppressed due to a deterioration of anelectroluminescent material.

Embodiment Mode 2

In this embodiment mode, a pixel that is included in a light emittingdevice of the present invention and that is different from the mode inFIG. 2 is described.

A pixel that is shown in FIG. 4A includes a light emitting element 404,a switching transistor 401, a driving transistor 402, a current controltransistor 403, and a transistor (an erasing transistor) 406 to erase anelectric potential of a video signal that is written in. The switchingtransistor 401 and the erasing transistor 406 each may be n-type orp-type. A capacitor element 405 can be provided for a pixel in additionto the above-mentioned elements. The driving transistor 402 and thecurrent control transistor 403 each have the same polarity. In thisembodiment mode, each of them has a p-type, but may have an n-type.Moreover, a threshold voltage, a value of L/W and an operating region ofthe driving transistor 402 and the current control transistor 403 may beset in the same way as the case of the embodiment mode 1.

A gate of the switching transistor 401 is connected to a first scanningline Gaj (j=1 to y). One of the source and the drain of the switchingtransistor 401 is connected to a signal line Si (i=1 to x), and theother is connected to each gate of the driving transistor 402 and thecurrent control transistor 403. Moreover, a gate of the erasingtransistor 406 is connected to a second scanning line Gej (j=1 to y).One of the source and the drain of the erasing transistor 406 isconnected to a power supply line Vi (i=1 to x), and the other one isconnected to each gate of the driving transistor 402 and of the currentcontrol transistor 403. The driving transistor 402 and the currentcontrol transistor 403 are connected in series. In addition, the drivingtransistor 402 and the current control transistor 403 are connected tothe power supply line Vi and the light emitting element 404 so that acurrent which is supplied from the power supply line Vi is supplied tothe light emitting element 404 as a drain current of the drivingtransistor 402 and the current control transistor 403. In FIG. 4A, asource of the current control transistor 403 is connected to the powersupply line Vi, and a drain of the driving transistor 402 is connectedto a pixel electrode of the light emitting element 404. A voltage isapplied from a power supply to each of a counter electrode of the lightemitting element 404 and the power supply line Vi so as to supply aforward bias current to the light emitting element 404. One of the twoelectrodes of the capacitor element 505 is connected to the power supplyline Vi, and the other is connected to each gate of the drivingtransistor 402 and of the current control transistor 403.

In the case where a source or the drain of the driving transistor 402 isconnected to an anode of the light emitting element 404, it is desirablethat the driving transistor 402 is a p-channel type transistor.Moreover, in the case where the source or the drain of the drivingtransistor 402 is connected to a cathode of the light emitting element404, it is desirable that the driving transistor 402 is an n-channeltype transistor.

The pixel shown in FIG. 4A can be described by classifying the operationinto a writing period, a storage period, and an erasing period.Operations of the switching transistor 401, the driving transistor 402,and the current control transistor 403 in the writing period and thestorage period are the same as in the case of FIG. 2. In the erasingperiod, the erasing transistor 406 turns ON when the second scanningline Gej is selected. Each electric potential of the power supply lineV1 to Vx is supplied to the gate of the driving transistor 402 and thecurrent control transistor 403 via the erasing transistor 406.Therefore, the current control transistor 403 turns OFF, and thus astate that the current is not compulsorily supplied to the lightemitting element 404 is made.

Next, another mode of a pixel that is included in a light emittingdevice of the present invention and that is different from the mode inFIG. 2 is described.

A pixel shown in FIG. 4B includes a light emitting element 414, aswitching transistor 411, a driving transistor 413, and a currentcontrol transistor 412. A capacitor element 415 may be provided in thepixel in addition to the above-mentioned elements. The drivingtransistor 413 and the current control transistor 412 each have the samepolarity. In this embodiment mode, each of them has a p-type, but mayhave an n-type. Moreover, a threshold voltage, a value of L/W and anoperating region of the driving transistor 413 and the current controltransistor 412 may be set in the same way as the embodiment mode 1. Inthe pixel shown in the FIG. 4B, the current control transistor 412 isprovided between the driving transistor 413 and the light emittingelement 414, which is different from FIG. 2. Thus, the current controltransistor 412 may be provided in a position in which a supply of adrain current of the driving transistor 413 to the light emittingelement 414 can be controlled.

Moreover, a device substrate corresponds to one mode before completing alight emitting element in a process of manufacturing a light emittingdevice of the present invention.

A transistor that is used in a light emitting device of the presentinvention may be a transistor which is formed by using a singlecrystalline silicon, a transistor using an SOI, or a thin filmtransistor using a polycrystalline silicon (polysilicon) or an amorphoussilicon. Moreover, a transistor using an organic semiconductor or atransistor using a carbon nanotube may be used. In addition, atransistor which is provided for the pixel of the light emitting deviceof the present invention may have a single gate structure, a double gatestructure or a multi gate structure which has more gate electrodes.

Embodiment 1

In this embodiment, one embodiment of a top view of the pixel shown inFIG. 4A is described. FIG. 5 shows a top view of a pixel of thisembodiment. The reference numeral 1001 corresponds to a signal line, thereference numeral 1002 corresponds to a power supply line, the referencenumeral 1004 corresponds to a first scanning line, and the referencenumeral 1003 corresponds to a second scanning line. In this embodiment,the signal line 1001 and the power supply line 1002 are formed from thesame conductive film, and the first scanning line 1004 and the secondscanning line 1003 are formed from the same conductive film. Inaddition, 1005 is a switching transistor, and a part of the firstscanning line 1004 functions as a gate electrode thereof. Moreover, thereference numeral 1006 is an erasing transistor, and a part of thesecond scanning line 1003 functions as a gate electrode thereof. Thereference numeral 1007 corresponds to a driving transistor, and thereference numeral 1008 corresponds to a current control transistor. Inthe driving transistor 1007, an active layer is wound so that the L/Wthereof is allowed to be higher than that of the current controltransistor 1008. The reference numeral 1009 corresponds to a pixelelectrode, and emits light in a region (a light emitting area) 1010 thatis overlapped with an electroluminescent layer and a cathode (both notshown in the figure).

Moreover, the top view of the present invention is only one embodiment,and it is needless to say that the present invention is not limited tothis.

Embodiment 2

In this embodiment, one embodiment of a structure of a drivingtransistor is described. FIG. 6 shows a cross-sectional view taken alongthe direction of a channel length of the driving transistor in thisembodiment. The driving transistor which is shown in FIG. 6 includes anactive layer 601, a gate insulating layer 602 which is in contact withthe active layer 601, and a gate electrode 603 which is overlapped withthe active layer 601 with the gate insulating layer 602 therebetween.Moreover, in FIG. 6, the gate electrode 603 is made up of one layer ofconductive film, but may be made up of two or more layers of conductivefilms.

The active layer 601 includes a channel forming region 604 which isoverlapped with the gate electrode 603 with the gate insulating layer602 therebetween, a source region 605 and a drain region 606 with thechannel forming region 604 therebetween, and an LDD region 607 whichexists between the source region 605 and the drain region 606 and thechannel forming region 604. In the present invention, a thresholdvoltage is controlled by adjusting a density of an impurity region whichis added to the channel forming region 604. Moreover, in the presentinvention, the driving transistor operates in the saturated region.

In the saturated region, a depletion layer of a drain joint portion isoverhanging, and an inversion layer (channel) is disappeared in a drainedge. A boundary point between a part in which the channel exists and apart in which the channel disappears is referred to as a pinch-offpoint. In addition, a carrier moves by being drawn from the pinch-offpoint to the drain region by a drain electric field. Therefore, a heightof the drain current is determined by the number of carriers that canpass through the channel, and a height of a potential barrier thatcarriers between the pinch-off point and the drain region have tosurmount.

When the pinch-off point exists in the channel forming region 604, theheight of the potential barrier depends on its crystallinity more thanan impurity concentration in the channel forming region 604. Therefore,when a crystallinity of a semiconductor film that is used for the activelayer varies, the amount of the drain current varies since the height ofthe potential barrier depends on its crystallinity. Consequently, inthis embodiment, the position is controlled by adjusting its drainvoltage and an impurity concentration in the LDD region or the like sothat the pinch-off point is formed in the LDD region 607 on the side ofthe drain region 606. The height of the potential barrier depends on theimpurity concentration in the LDD region more than its crystallinity ofthe semiconductor film since the pinch-off point is in the LDD region607 on the side of the drain region 606. Therefore, the variation of thedrain current due to a crystalline variation can be controlled.

Embodiment 3

In this embodiment, structures of a signal line driver circuit and ascanning line driver circuit that are used for a light emitting deviceof the present invention are described. FIG. 7A is a block diagram of asignal line driver circuit 701, which includes a shift register 702, alatch A 703, and a latch B 704. In the signal line driver circuit 701, aclock signal (CLK) and a start pulse (SP) are inputted to the shiftregister 702. The shift register 702 sequentially generates a timingsignal based on the clock signal (CLK) and the start pulse (SP). Then,the timing signal is supplied to a circuit in the succeeding stage inseries through a buffer (not shown in the figure) or the like.

The timing signal from the shift register 702 is buffer-amplified by thebuffer or the like. A load capacitance (parasitic capacitance) is largesince a large number of circuits and elements are connected to a wiringto which the timing signals are supplied. The buffer is provided inorder to prevent “dullness” in the rise and fall of the timing signaldue to the large load capacitance. In addition, the buffer is notnecessary provided. The timing signal that is buffer-amplified by thebuffer is supplied to the latch A 703. The latch A 703 has a pluralityof latch stages for processing an n-bit digital video signal. The latchA 703 takes in and stores an n-bit digital video signal in series whichis supplied from external of the signal line driver circuit 701 when theabove-mentioned timing signal is inputted

When the video signal is taken in the latch A 703, the video signal maybe sequentially inputted into the plurality of stages in the latch A703. However, the present invention is not limited to this structure. Aso-called division driving may be performed, in which the latches in theplurality of stages in the latch A 703 are divided into several groupsand the video signal is inputted to every group in parallel andsimultaneously. At this time, the number of the groups is referred to asthe division number. For example, in the case where the latches aredivided into four groups, it can be said as driving through thefour-division. When data of the latch A703 is written in all latches, alatch signal is supplied to the latch B 704. At this moment, the videosignal that is written and stored in the latch A 703 is simultaneouslysent to the latch B 704 of all stages and is written therein. The periodwhen a data is sent from the latch A 703 to the latch B 704 is called alatch period.

The video signal is written again into the latch A 703 that has finishedsending the video signal to the latch B 704, which is performed based onthe timing signal from the shift register 702. During one line period inthe second turn, the video signal that has been written again into andstored in the latch B 704 is inputted to the signal line.

FIG. 7B is a block diagram that shows a structure of a scanning linedriver circuit. A scanning line driver circuit 705 includes a shiftregister 706 and a buffer 707 respectively. In some cases, a levelshifter may be included. In the scanning line driver circuit 705, atiming signal from the shift register 706 is supplied to the buffer 707,and is supplied to a corresponding scanning line (or a first scanningline, a second scanning line). Gates of switching transistors (or anerasing transistors) of pixels of one line is connected to the scanningline. Moreover, a buffer that can pass much current is used sinceswitching transistors (or an erasing transistor) of pixels per one linemust turn ON simultaneously.

Embodiment 4

In this embodiment, an external appearance of a light-emitting device ofthe present invention is described with reference to FIG. 8. FIG. 8 is atop view of the light emitting device which is formed by sealing adevice substrate provided with a transistor by using a sealing member.FIG. 8B is a cross-sectional view taken along A-A′ of FIG. 8A, and FIG.8C is a cross-sectional view taken along B-B′ of FIG. 8A.

A seal member 4009 is provided to surround a pixel portion 4002, asignal line driver circuit 4003, and scanning line driver circuits 4000a, 4000 b, all of which are provided over a substrate 4001. Further, acover member 4008 is provided over the pixel portion 4002, the signalline driver circuit 4003, and the scanning line driver circuits 4004 a,4004 b. Thus, the pixel portion 4002, the signal line driver circuit4003, and the scanning line driver circuits 4004 a, 4004 b are sealedwith a filler 4210 by the substrate 4001, the seal member 4009 and thecover member 4008.

Further, the pixel portion 4002, the signal line driver circuit 4003,and the scanning line driver circuits 4004 a, 4004 b, which are providedover the substrate 4001, have a plurality of transistors. In FIG. 8B, adriving transistor (note that an n-channel transistor and a p-channeltransistor are illustrated here) 4201 which is included in the signalline driver circuit 4003 and a transistor 4202 which is included in thepixel portion 4002, which are formed over a base film 4010, are shown,typically.

An interlayer insulating film (a leveling film) 4301 is formed over thedriving transistor 4201 and the transistor 4202, and an anode (anode)4203 that is electrically connected to a drain of the transistor 4202 isformed thereon. A transparent conductive film having a large workfunction is used for the anode 4203. A compound of indium oxide and tinoxide, a compound of indium oxide and zinc oxide, zinc oxide, tin oxideor indium oxide can be used for the transparent conductive film. Thetransparent conductive film may be added with gallium.

Then, an insulating film 4302 is formed over the anode 4203, and in theinsulating film 4302, an opening portion is formed over the anode 4203.In this opening portion, an electroluminescent layer 4204 is formed overthe anode 4203. A known organic electroluminescent material or aninorganic electroluminescent material can be used for theelectroluminescent layer 4204. Further, either a low molecule weightcompound series (monomer series) or a high molecule weight compoundseries (polymer series) may be used as the organic electroluminescentmaterial. A known vapor deposition technique or an application methodtechnique may be used as a method of forming the electroluminescentlayer 4204. Further, the structure of the electroluminescent layer mayemploy a lamination structure or a single layer structure by freelycombining a hole injection layer, a hole transporting layer, a lightemitting layer, an electron transporting layer, or an electron injectionlayer.

A cathode 4205 that is made of a conductive film having light-shieldingproperty (typically, a conductive film containing aluminum, copper orsilver as its main constituent or a lamination film of the conductivefilm and another conductive film) is formed over the electroluminescentlayer 4204. Moreover, it is desirable that moisture or oxygen that existon an interface between the cathode 4205 and the electroluminescentlayer 4204 are removed as much as possible. Therefore, such a device isnecessary that the electroluminescent layer 4204 is formed in nitrogenor a rare gas atmosphere, and then, the cathode 4205 is formed withoutexposure to oxygen or moisture. In this embodiment, the above-mentionedfilm formation is possible by using a multi-chamber type (cluster tooltype) film formation device. Moreover, a predetermined voltage isapplied to the cathode 4205.

As described above, a light emitting element 4303 that includes theanode 4203, the electroluminescent layer 4204 and the cathode 4205 isformed. Further, a protective film 4209 is formed over the insulatingfilm 4302 to cover the light emitting element 4303. The protective film4209 is effective in preventing oxygen, moisture, or the like fromintruding into the light emitting element 4303.

Reference numeral 4005 a denotes a lead wiring that is connected to apower supply line, and is electrically connected to a source of thetransistor 4202. The lead wiring 4005 a is led between the seal member4009 and the substrate 4001, and is electrically connected to an FPCwiring 4301 of an FPC 4006 via an anisotropic conductive film 4300.

As the cover member 4008, a glass member, a metal member (typically,stainless member), a ceramics member or a plastic member (including aplastic film) can be used. As the plastic material, an FRP(Fiberglass-Reinforced Plastics) plate, a PVF (polyvinyl fluoride) film,a Mylar film, a polyester film, or an acrylic resin film can be used.Further, a sheet with a structure in which an aluminum foil issandwiched with a PVF film or a Mylar film can be also used.

However, in the case where light is emitted in the direction of thecover member, the cover member needs to be transparent. In this case, atransparent substance such as a glass plate, a plastic plate, apolyester film or an acrylic film is used.

Further, in addition to an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin can be used as thefiller 4210, and PVC (polyvinyl chloride), acrylic, polyimide, epoxyresin, silicon resin, PVB (polyvinyl butyral) or EVA (ethylene vinylacetate) can be used. In this embodiment, nitrogen is used as thefiller.

Moreover, a concave portion 4007 is provided on the face of the covermaterial 4008 on the substrate 4001 side, and a hygroscopic substance ora substance 4207 that can absorb oxygen is arranged therein in orderthat the filler 4210 is exposed to the hygroscopic substance(preferably, barium oxide) or the substance that can absorb oxygen.Then, the hygroscopic substance or the substance that can absorb oxygen4207 is held in the concave portion 4007 by a concave portion covermember 4208 such that the hygroscopic substance or the substance thatcan absorb oxygen 4207 is not scattered. Note that the concave portioncover member 4208 has a fine mesh form, and has a structure in which airor moisture is penetrated while the hygroscopic substance or thesubstance that can absorb oxygen 4207 is not penetrated. Thedeterioration of the light emitting element 4303 can be suppressed byproviding the hygroscopic substance or the substance that can absorboxygen 4207.

As shown in FIG. 8C, the anode 4203 is formed, and at the same time, aconductive film 4203 a is formed so as to be in contact with the leadwiring 4005 a. Further, the anisotropic conductive film 4300 has aconductive filler 4300 a. The conductive film 4203 a over the substrate4001 and the FPC wiring 4301 a over the FPC 4006 are electricallyconnected to each other by the conductive filler 4300 a by heat-pressingthe substrate 4001 and the FPC 4006.

Embodiment 5

A light emitting device using a light emitting element is aself-luminous type, and thus exhibits more excellent visibility in alight place, and further has a wider viewing angle as compared to aliquid crystal display device. Therefore, the light emitting device canbe applied to a display portion in various kinds of electronic devices.

The electronic devices using a light emitting device of the presentinvention include a video camera, a digital camera, a goggle typedisplay (head mounted display), a navigation system, a soundreproduction device (a car audio equipment, an audio component or thelike), a lap-top computer, a game machine, a portable informationterminal (a mobile computer, a mobile phone, a portable game machine, anelectronic book, and the like), an image reproducing device providedwith recording medium (typically, a device provided with a display thatcan reproduce a recording medium such as DVD: digital versatile disc)and display the image) and the like. Especially, it is desirable thatthe light emitting device is employed for the portable informationterminal whose display is watched from an oblique direction, since awidth of a viewing angle is emphasized in the portable informationterminal.

FIG. 9A shows a display device which includes a casing 2001, a supporttable 2002, a display portion 2003, a speaker portion 2004, a videoinput terminal 2005 and the like. The display device of the presentinvention is completed by using the light-emitting device of the presentinvention for the display portion 2003. The light emitting device is theself-luminous type and thus requires no backlight. Therefore, the lightemitting device can have a thinner display portion than that of theliquid crystal display device. Note that the light emitting elementdisplay device includes all display devices for displaying information,for example, a personal computer, a receiver of TV broadcasting and anadvertising display.

FIG. 9B shows a digital still camera which includes a main body 2101, adisplay portion 2102, an image receiving portion 2103, an operation key2104, an external connection port 2105, a shutter 2106, and the like.The digital still camera of the present invention is completed by usingthe light emitting device of the present invention for the displayportion 2102.

FIG. 9C shows a laptop computer which includes a main body 2201, acasing 2202, a display portion 2203, a keyboard 2204, an externalconnection port 2205, a pointing mouse 2206, and the like. The laptopcomputer of the present invention is completed by using the lightemitting device of the present invention for the display portion 2203.

FIG. 9D shows a mobile computer which includes a main body 2301, adisplay portion 2302, a switch 2303, an operation key 2304, an infraredport 2305, and the like. The mobile computer of the present invention iscompleted by using the light emitting device of the present inventionfor the display portion 2302.

FIG. 9E shows a portable image reproducing device provided withrecording medium (specifically, a DVD reproduction device), whichincludes a main body 2401, a casing 2402, a display portion A 2403, adisplay portion B 2404, a recording medium (DVD or the like) readingportion 2405, an operation key 2406, a speaker portion 2407 and thelike. The display portion A 2403 is used mainly for displaying imageinformation, and the display portion B 2404 is used mainly fordisplaying character information. In addition, the image reproducingdevice provided with recording medium further includes a home video gamemachine and the like. The image reproducing device of the presentinvention is completed by using the light emitting device of the presentinvention for the display portions A 2403 and B 2404.

FIG. 9F shows a goggle type display (head mounted display) whichincludes a main body 2501, a display portion 2502, an arm portion 2503and the like. The goggle type display of the present invention iscompleted by using the light emitting device of the present inventionfor the display portion 2502.

FIG. 9G shows a video camera which includes a main body 2601, a displayportion 2602, a casing 2603, an external connection port 2604, a remotecontrol receiving portion 2605, an image receiving portion 2606, abattery 2607, a sound input portion 2608, an operation key 2609, an eyepiece 2610, and the like. The video camera of the present invention iscompleted by using the light emitting device of the present invention tothe display portion 2602.

FIG. 9H shows a mobile phone which includes a main body 2701, a casing2702, a display portion 2703, a sound input portion 2704, a sound outputportion 2705, an operation key 2706, an external connection port 2707,an antenna 2708, and the like. Note that, in the display portion 2703,consumption current of the mobile phone can be suppressed by displayingwhite-colored characters on a black-colored background. The mobile phoneof the present invention is completed by using the light emitting deviceof the present invention for the display portion 2703.

In addition, when luminance of light emitted from an organicelectroluminescent material becomes higher in the future, thelight-emitting device can be used for a front or a rear projector byenlarging and projecting light that contains outputted image informationthrough a lens or the like.

The above-mentioned electronic device is more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system) or the like, and inparticular, has more opportunities of displaying moving pictureinformation. The light emitting device is suitable for displaying movingpictures since the response speed of an organic electroluminescentmaterial is much faster.

In addition, a portion that is emitting light consumes power in thelight emitting device, thus it is desirable to display information insuch a manner that the light emitting portion therein becomes as smallas possible. Accordingly, in the case where the light emitting device isapplied to a display portion which mainly displays character informationlike a portable information terminal, and particularly, a mobile phoneor a sound reproduction device, it is desirable to drive it so that thecharacter information is shown in a light emitting portion by using anon-emission portion as the background.

As described above, the range in which the present invention is appliedis extremely wide, and can be applied to electronic devices in allfields. The electronic device in this embodiment may employ a lightemitting device having any one of structures shown in Embodiments 1through 6.

Embodiment 6

A transistor used in the present invention may be formed by usingamorphous silicon. In the case where the transistor is formed by usingamorphous silicon, a manufacturing method can be simplified since acrystallization process is dispensed with, and thus the cost reductioncan be achieved. The transistor that is formed by using amorphoussilicon is preferably not p-type but n-type, since the n-type has highermobility and thus is more suitably applied to a pixel of a lightemitting device than a p-type. In this embodiment, a cross-sectionalstructure of the pixel in the case where a driving transistor and acurrent control transistor are both n-channel transistors is described.

FIG. 11A shows a cross-sectional view of a pixel in the case where adriving transistor 6001 is n-type and light emitted from a lightemitting element 6002 passes through an anode 6005 side. In FIG. 11A, acathode 6003 of the light emitting element 6002 is electricallyconnected to the driving transistor 6001, and an electroluminescentlayer 6004 and the anode 6005 are sequentially laminated over thecathode 6003. A known material can be used for the cathode 6003 as longas it is a conductive film which has a small work function and reflectslight. For example, Ca, Al, CaF, MgAg, AlLi, and the like are desirablyused. The electroluminescent layer 6004 may be structured by a singlelayer or a lamination of multiple layers. In the case where the layer isstructured by multiple layers, an electron injection layer, an electrontransporting layer, a light emitting layer, a hole transporting layer,and a hole injection layer are sequentially laminated over the cathode6003. Note that not all of the layers are necessarily provided. Theanode 6005 is formed from a transparent conductive film which transmitslight, and for example, a transparent conductive film in which 2 to 20%zinc oxide (ZnO) is mixed with indium oxide may be used, in addition toan ITO.

A portion where the cathode 6003, the electroluminescent layer 6004 andthe anode 6005 are overlapped corresponds to the light emitting element6002. In the case of the pixel shown in FIG. 11A, light emitted from thelight emitting element 6002 pass through the anode 6005 side as shown bythe outline arrow.

FIG. 11B shows a cross-sectional view of a pixel in the case where adriving transistor 6011 is n-type and light from a light emittingelement 6012 is emitted to a cathode 6013 side. In FIG. 11B, the cathode6013 of the light emitting element 6012 is formed over a transparentconductive film 6017 which is electrically connected to the drivingtransistor 6011, and an electroluminescent layer 6014 and an anode 6015are sequentially laminated over the cathode 6013. A shielding film 6016that reflects or shuts of light is formed to cover the anode 6015. As isthe case with FIG. 11A, a known material can be used for the cathode6013 as long as it is a conductive film having a small work function,and the film is formed to be thin enough to transmit light. For example,Al having a thickness of 20 nm can be used for the cathode 6013. Theelectroluminescent layer 6014 may be structured by a single layer or alamination of multiple layers, as is the case with FIG. 11A. The anode6015 can be formed of a transparent conductive film, as is the case withFIG. 11A, although it is not required to transmit light. Alight-reflective metal can be used for the shielding film 6016, forexample. However, the film is not limited to a metal film. For example,a resin doped with black pigment, or the like can be used.

A portion where the cathode 6013, the electroluminescent layer 6014, andthe anode 6015 are overlapped corresponds to the light emitting element6012. In the case of the pixel shown in FIG. 11B, light which is emittedfrom the light emitting element 6012 pass through the cathode 6013 sideas shown by the outline arrow.

It is to be noted that a current controlling transistor may be connectedbetween the driving transistor and the light emitting element, althoughan example in which the driving transistor is electrically connected tothe light emitting element is shown in this embodiment.

Embodiment 7

In this embodiment, a cross-sectional view of a pixel in the case wherea driving transistor and a current control transistor are p-type isdescribed.

FIG. 12A shows a cross-sectional view of a pixel in which a drivingtransistor 6021 is p-type and light which is emitted from a lightemitting element 6022 passes through an anode 6023 side. In FIG. 12A,the anode 6023 of the light emitting element 6022 is electricallyconnected to the driving transistor 6021, and an electroluminescentlayer 6024 and a cathode 6025 are sequentially laminated over the anode6023. A known material can be used for the cathode 6205 as long as it isa conductive film which has a small work function and reflects light.For example, Ca, Al, CaF, MgAg, AlLi, and the like are desirably used.The electroluminescent layer 6024 may be structured by a single layer ora lamination of multiple layers. In the case where the layer isstructured by multiple layers, a hole injection layer, a holetransporting layer, a light emitting layer, an electron transportinglayer, and an electron injection layer are sequentially laminated overthe anode 6023. Note that not all of the layers are necessarilyprovided. The anode 6023 is formed from a transparent conductive filmthat transmits light, and for example, a transparent conductive film inwhich 2 to 20% zinc oxide (ZnO) is mixed with indium oxide may be used,in addition to an ITO.

A portion where the anode 6023, the electroluminescent layer 6024, andthe cathode 6025 are overlapped corresponds to the light emittingelement 6022. In the case of the pixel shown in FIG. 12A, light which isemitted from the light emitting element 6022 pass through the anode 6023side as shown by the outline arrow.

FIG. 12B shows a cross-sectional view of a pixel in which a drivingtransistor 6031 is p-type and light which is emitted from a lightemitting element 6032 passes through an cathode 6035 side. In FIG. 12B,an anode 6033 of the light emitting element 6032 is formed over a wiring6037 which is electrically connected to the driving transistor 6031, andan electroluminescent layer 6034 and a cathode 6035 are sequentiallylaminated over the anode 6033. According to the above-mentionedstructure, the light is reflected on the wiring 6037 even when light isreflected off the anode 6033. As is the case with FIG. 12A, a knownmaterial can be used for the cathode 6035 as long as it is a conductivefilm having a small work function, and the film is formed to be thinenough to transmit light. For example, Al having a thickness of 20 nmcan be used for the cathode 6035. As is the case with FIG. 12A, theelectroluminescent layer 6034 may be structured by a single layer or alamination of multiple layers. The anode 6035 can be formed from atransparent conductive film, as is the case with FIG. 12A, although itis not required to transmit light.

A portion where the anode 6033, the electroluminescent layer 6034, andthe cathode 6035 are overlapped corresponds to the light emittingelement 6032. In the case of the pixel shown in FIG. 12B, light which isemitted from the light emitting element 6032 pass through the cathode6035 side as shown by the outline arrow.

It is to be noted that a current control transistor may be interposedbetween a driving transistor and a light emitting element, although anexample in which the driving transistor is electrically connected to thelight emitting element is shown in this embodiment.

Embodiment 8

In this embodiment, a cross-sectional structure of a pixel in the casewhere a driving transistor and a current control transistor are bothbottom-gate types is described.

FIG. 13A shows a cross-sectional view of a pixel of this embodiment. Thereference numeral 6501 corresponds to a driving transistor, and thereference numeral 6502 corresponds to a current control transistor. Thedriving transistor 6501 includes a gate electrode 6503 which is formedover a substrate 6500 having an insulating surface, a gate insulatingfilm 6504 which is formed over the substrate 6500 so as to cover thegate electrode 6503, and a semiconductor film 6505 which is formed overa position which is overlapped with the gate electrode 6503 with thegate insulating film 6504 therebetween. The semiconductor film 6505includes two impurity regions 6506 a and 6506 b to which an impuritythat gives a conductive type is added and which function as a source ora drain. In addition, the impurity region 6506 a is connected to awiring 6508.

Like the driving transistor 6501, the current control transistor 6502includes a gate electrode 6510 which is formed over the substrate 6500having an insulating surface, the gate insulating film 6504 which isformed over the substrate 6500 so as to cover the gate electrode 6510,and a semiconductor film 6511 which is formed over a position which isoverlapped with the gate electrode 6510 with the gate insulating film6504 therebetween. The semiconductor film 6511 includes two impurityregions 6512 a and to which an impurity that gives a conductive type isadded and which function as a source or a drain and. In addition, theimpurity region 6512 a is connected to the impurity region 6506 b thatis included in the driving transistor 6501 via a wiring 6513.

Both of the driving transistor 6501 and the current control transistor6502 are covered with a protective film 6507 that is made of aninsulating film. In addition, the wiring 6508 is connected to an anode6509 via a contact hole that is formed in the protective film 6507.Moreover, the driving transistor 6501, the current control transistor6502 and the protective film 6507 are covered with an interlayerinsulating film 6520. The interlayer insulating film 6520 has an openingportion, and the anode 6509 is exposed in the opening portion. Anelectroluminescent layer 6521 and a cathode 6522 are formed over theanode 6509.

In FIG. 13A, a threshold voltage is controlled by adding an impuritythat gives n-type conductivity to a channel forming region of thesemiconductor film 6505 which is included in the driving transistor 6501so that the driving transistor 6501 is allowed to be normally-on(depletion type). Note that the current control transistor 6502 isnormally-off (enhancement type).

FIG. 13B shows a cross-sectional view of a pixel of this embodimentwhich is different from FIG. 13A. The reference numeral 6601 correspondsto a driving transistor, and the reference numeral 6602 corresponds to acurrent control transistor. The structures of the driving transistor6601 and the current control transistor 6602 are similar to the case ofFIG. 13A. However, in FIG. 13B, the driving transistor 6601 is formedover an interlayer insulating film 6603 which covers the current controltransistor 6602. In addition, the driving transistor 6601 and thecurrent control transistor 6602 are electrically connected with a wiring6604 via a contact hole which is formed in the interlayer insulatingfilm 6603.

In FIG. 13B, a semiconductor film 6605 included in the drivingtransistor 6601 controls a threshold voltage to be normally-on(depletion type) by adding an impurity that gives an n-type in filmforming.

Note that, in FIG. 13A and FIG. 13B, the case where the drivingtransistor and the current control transistor are each n-type isdescribed, however, they may be p-type. In this case, p-type is used foran impurity to control a threshold of the driving transistor.

Embodiment 9

This embodiment describes an exterior appearance of a light emittingdevice of the present invention in which a transistor formed fromamorphous silicon is used for a pixel portion. In this embodiment, asignal line driver circuit or a scanning line driver circuit whichsupplies various signals to the pixel portion is manufactured separatelyfrom a panel, and are mounted on the panel by using an FPC or the like.FIG. 14A shows a top view of the panel of this embodiment, and FIG. 14Bshows a cross-sectional view taken along A-A′ of FIG. 14A.

In a panel that is shown in FIGS. 14A and 14B, the pixel portion 5002which is provided over a substrate 5001 is surrounded by a seal member5009 which is provided over the substrate 5001 likewise, and sealedbetween the substrate 5001 and a cover member 5008 together with afiller 5210.

A plurality of transistors and a light emitting element 5303 are formedin the pixel portion 5002. In FIG. 14B, a driving transistor 5202 thatis included in the pixel portion 5002 is representatively shown. Thedriving transistor 5202 and the light emitting element 5303 iselectrically connected. Note that an example that the driving transistorand the light emitting element is electrically connected is shown inthis embodiment, but the current control transistor may be connected inseries between the driving transistor and the light emitting element.

The driving transistor 5202 and the light emitting element 5303 arecovered with the filler 5210. In this embodiment, a resin to which ahygroscopic substance such as barium oxide is added with is used as thefiller 5210. An ultraviolet curable resin or a thermosetting resin canbe used as the resin, and PVC (polyvinyl chloride), acrylic, polyimide,epoxy resin, silicone resin, PVB (polyvinyl butyral) or EVA (ethylenevinyl acetate) can be used. Note that an inert gas such as nitride orargon can be used as the filler 5210.

The reference numeral 5010 is a lead wiring that is connected to a powersupply, and that is electrically connected to a source of the drivingtransistor 5202. The lead wiring 5010 passes through between the sealmember 5009 and the substrate 5001, and is electrically connected to anFPC wiring 5301 of an FPC 5006, via an anisotropic conductive film 5300.

Embodiment 10

A pixel shown in FIG. 15 includes a light emitting element 804, aswitching transistor 801, a driving transistor 802, a current controltransistor 803, and an erasing transistor 806. In addition to theabove-mentioned elements, a capacitor element 805 may be provided in thepixel. The driving transistor 802, the current control transistor 803,and the erasing transistor 806 each have the same polarity. In thisembodiment, each of them has a p-type, but they may have an n-type. Athreshold voltage, a value of L/W and an operating region of the drivingtransistor 802 and the current control transistor 803 may be set in thesame way as the case of the embodiment 1.

A gate of the switching transistor 801 is connected to a first scanningline Gaj (j=1 to y). One of the source and the drain of the switchingtransistor 801 is connected to a signal line Si (i=1 to x), and theother is connected to each gate of the driving transistor 802 and thecurrent control transistor 803. In addition, a gate of the erasingtransistor 806 is connected to a second scanning line Gej (j=1 to y).

The driving transistor 802, the current control transistor 803 and theerasing transistor 806 are connected in series. In addition, the drivingtransistor 802, the current control transistor 803 and the erasingtransistor 806 are connected to a power supply line Vi (i=1 to x) andthe light emitting element 804 so that a current which is supplied fromthe power supply line Vi is supplied to the light emitting element 804as a drain current of the driving transistor 802, the current controltransistor 803 and the erasing transistor 806. In FIG. 15, a source ofthe erasing transistor 806 is connected to the power supply line Vi, adrain of the driving transistor 802 is connected to an electrode of thelight emitting element 804, and the current control transistor 803 isprovided between the erasing transistor 806 and the driving transistor802.

Note that the driving transistor 802, the current control transistor 803and the erasing transistor 806 may be connected in series between thepower supply line Vi and the light emitting element 804, and thepositional relation of the three transistors are not limited to astructure shown in FIG. 15. For example, the erasing transistor 806 maybe provided between the driving transistor 802 and the current controltransistor 803, or may be provided at a closer position to the lightemitting element 804 than the driving transistor 802 and the currentcontrol transistor 803.

A voltage is applied from a power supply to each of a counter electrodeof the light emitting element 804 and the power supply line Vi so as tosupply a forward bias current to the light emitting element 804. One ofthe two electrodes of the capacitor element 805 is connected to thepower supply line Vi, and the other is connected to each gate of thedriving transistor 802 and the current control transistor 803.

The operation of the pixel shown in FIG. 15 can be described byclassifying the operation into a writing period, a storage period, andan erasing period. Operations of the switching transistor 801, thedriving transistor 802, and the current control transistor 803 in thewriting period and the storage period are the same as in the case ofFIG. 2. However, the erasing transistor 806 controls an electricpotential of the second scanning line Gej so that the erasing transistor806 turns ON in the writing period and the storage period, and turns OFFin the erasing period. When the erasing transistor 806 turns OFF in theerasing period, a state that a current is not forcibly supplied to thelight emitting element 804 can be obtained.

Embodiment 11

A cross-sectional structure of a pixel of a light emitting element ofthe present invention is described with reference to FIG. 16. In FIG.16, a transistor 7001 is formed over a substrate 7000. The transistor7001 is covered with a first interlayer insulating film 7002, and acolor filter 7003 which is formed from a resin or the like and a wiring7004 which is electrically connected to the transistor 7001 via acontact hole are formed in the first interlayer insulating film 7002.

Moreover, a second interlayer insulating film 7005 is formed over thefirst interlayer insulating film 7002 to cover the color filter 7003 andthe wiring 7004. Note that the first interlayer insulating film 7002 orthe second interlayer insulating film 7005 is formed with a single layeror a laminated layer of a silicon oxide film, a silicon nitride film, ora silicon oxynitride film by plasma CVD or sputtering. A film that asilicon oxynitride film in which mole fraction of oxygen is higher thanthat of nitrogen is laminated over a silicon oxynitride film in whichmole fraction of nitrogen is higher than that of oxygen, and may be usedas the first interlayer insulating film 7002 or the second interlayerinsulating film 7005. Alternatively, an organic resin film may be usedas the first interlayer insulating film 7002 or the second interlayerinsulating film 7005.

A wiring 7006 that is electrically connected to the wiring 7004 via acontact hole is formed in the second interlayer insulating film 7005. Aportion of the wiring 7006 has a function of anode, and is formed in aposition that is overlapped with the color filter 7003 with the secondinterlayer insulating film 7005 therebetween.

In addition, an organic resin film 7008 that is used as a barrier isformed over the second interlayer insulating film 7005. The organicresin film 7008 has an opening portion, and a light emitting element7011 is formed by overlapping the wiring 7006, an electroluminescentlayer 7009 and a cathode 7010 with one another in the opening portion.The electroluminescent layer 7009 has a structure of a single layer of alight emitting layer or a laminated structure of a plurality of layersincluding the light emitting layer. Note that a protective film may beformed over the organic resin film 7008 and the cathode 7010. In thiscase, a film that is less permeable to a substance that promotes adeterioration of a light emitting element, such as moisture or oxide, ascompared with other insulating film, is used. Representatively, it isdesirable to use a DLC film, a carbon nitride film, and a siliconnitride film that is formed by RF sputtering or the like. Moreover, itis possible to use as a protective film by laminating a film that isless permeable to the above-mentioned substance such as moisture oroxide, and a film that is more permeable to the substance such asmoisture or oxide than the film.

Moreover, the organic resin film 7008 is heated under vacuum atmosphereto remove absorbed moisture, oxygen, or the like before theelectroluminescent layer 7009 is formed. Specifically, the heattreatment is carried out under vacuum atmosphere within the range of100° C. to 200° C., and for about a half hour to 1 hour. It ispreferable to be equal to or less than 3×10⁻⁷ Torr, and if possible,being equal to or less than 3×10⁻⁸ Torr is most preferable. Moreover, inthe case where the electroluminescent layer is formed after the heattreatment for the organic resin is performed on under vacuum atmosphere,the reliability can be further enhanced by keeping vacuum atmospherejust before forming the film.

In addition, as for an edge portion of the opening portion of theorganic resin film 7008, the electroluminescent layer 7009 that isoverlapped partly with the organic resin film 7008 is desirable to beformed roundish so that a hole is not made in the edge portion.Specifically, a curvature radius of a curve which is drawn in a crosssection of the organic resin film in the opening portion is desirably inthe range of approximately 0.2 to 2 μm.

According to the above-mentioned structure, coverage of anelectroluminescent layer and a cathode that are formed later can beenhanced. Thus, it can be prevented that the wiring 7006 and the cathode7010 are short-circuited in the holes that are formed in theelectroluminescent layer 7009. Moreover, by relieving stress of theelectroluminescent layer 7009, a defect called shrink, in which a lightemitting region decreases, can be reduced and the reliability is thusenhanced.

Note that FIG. 16 shows an example in which a positive photosensitiveacryl resin is used as the organic resin film 7008. The photosensitiveorganic resin is classified into the positive type in which the regionexposed to the energy line such as light, electron, ion, or the like isremoved, and the negative type in which the exposed region is left. Inthe present invention, the organic resin film of the negative type maybe used. Moreover, the organic resin film 7008 may be formed from thephotosensitive polyimide. In the case where the organic resin film 7008is formed from the negative type acrylic, the edge portion of theopening portion becomes an S-like cross section. On this occasion, it isdesirable that the curvature radius in the upper edge portion and thelower edge portion of the opening portion is in the range of 0.2 to 2μm.

The wiring 7006 can be formed by using a transparent conductive film. Atransparent conductive film in which 2 to 20% zinc oxide (ZnO) is mixedwith indium oxide may be used, in addition to an ITO. In FIG. 16, an ITOis used as the wiring 7006. The wiring 7006 may be polished by CMPmethod or by cleaning with porous body of polyvinyl alcohols so that thesurface of the wiring 7006 is flattened. Moreover, the surface of thewiring 7006 may be irradiated with ultraviolet ray or may be processedwith oxygen plasma after being polishing with the CMP method.

In addition, the cathode 7010 is formed to be thin enough to transmitlight. Any known material can be used for the cathode 7010 as long as itis a conductive film having a small work function. For example, Ca, Al,CaF, MgAg, AlLi, and the like are preferably used. Note that there isalso a method of employing an ITO that has a small work function byadding Li, instead of a method of thinning the film, to obtain lightfrom the cathode side. A light emitting element of the present inventionmay have a structure that light is emitted from both sides of the anodeand the cathode.

Note that, practically, when the device in FIG. 16 has been completed, aprotective film (a laminated film, an ultraviolet curable resin, or thelike) having high air tightness and less degasification or alight-transmitting cover member 7012 is preferably used for packaging(sealing) the device, so as not to be further exposed to the air. Atthis moment, the reliability of the OLED is enhanced by filling insideof the cover member with an inert atmosphere or providing a hygroscopicmember inside (for example, barium oxide). Moreover, in the presentinvention, a color filter 7013 may be provided for the cover member7012.

Note that the present invention is not limited to the above-mentionedmanufacturing method, and a known method can be used as well.

Embodiment 12

In this embodiment, one embodiment of a top view of a pixel shown inFIG. 4A is described. FIG. 17 shows a top view of a pixel of thisembodiment. The reference numeral 8001 corresponds to a signal line, thereference numeral 8002 corresponds to a power supply line, the referencenumeral 8004 corresponds to a first scanning line, the reference numeral8003 corresponds to a second scanning line. In this embodiment, thesignal line 8001 and the power supply line 8002 are formed from the sameconductive film, and the first scanning line 8004 and the secondscanning line 8003 are formed from the same conductive film. Inaddition, the reference numeral 8005 is a switching transistor, and aportion of the first scanning line 8004 functions as the gate electrodethereof. Moreover, the reference numeral 8006 is an erasing transistor,and a portion of the second scanning line 8003 functions as the gateelectrode thereof. The reference numeral 8007 corresponds to a drivingtransistor, and the reference numeral 8008 corresponds to a currentcontrol transistor. In the driving transistor 8007, an active layer iswound so that the L/W thereof is allowed to be higher than that of thecurrent control transistor 8008. The reference numeral 8009 correspondsto a pixel electrode, and emits light in a region (a light emittingarea) 8010 that is overlapped with an electroluminescent layer and acathode (both not shown in the figure).

Moreover, the top view of the present invention is only one embodiment,and it is needless to say that the present invention is not limited tothis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Diagrams describing an operating area of a transistor which isincluded in a pixel of the present invention.

FIG. 2 A circuit diagram of a pixel of a light emitting device of thepresent invention.

FIG. 3 Diagrams showing an operation of a pixel shown in FIG. 2.

FIG. 4 Circuit diagrams of a pixel of a light emitting device of thepresent invention.

FIG. 5 A top view of a pixel of a light emitting device of the presentinvention.

FIG. 6 A diagram showing a cross-sectional structure of a drivingtransistor.

FIG. 7 Diagrams showing structures of a driver circuit of a lightemitting device.

FIG. 8 A top view and cross-sectional views of a light emitting deviceof the present invention.

FIG. 9 Views of electronic devices using a light emitting device of thepresent invention.

FIG. 10 A circuit diagram of a pixel of a conventional light emittingdevice.

FIG. 11 Cross-sectional views of a pixel of a light emitting device ofthe present invention.

FIG. 12 Cross-sectional views of a pixel of a light emitting device ofthe present invention.

FIG. 13 Cross-sectional views of a pixel of a light emitting device ofthe present invention.

FIG. 14 A top view and a cross-sectional view of a light emitting deviceof the present invention.

FIG. 15 A circuit diagram of a pixel of a light emitting device of thepresent invention.

FIG. 16 A cross-sectional view of a pixel of a light emitting device ofthe present invention.

FIG. 17 A top view of a pixel of a light emitting device of the presentinvention.

1. A light emitting device comprising: a light emitting element and afirst transistor and a second transistor each for controlling current tobe supplied to the light emitting element, which are formed in a pixel,wherein a threshold voltage of the first transistor is lower than athreshold voltage of the second transistor, wherein a channel length ofthe first transistor is longer than a channel width thereof, wherein achannel length of the second transistor is equal to or shorter than achannel width thereof, wherein gate electrodes of the first transistorand the second transistor are connected to each other, wherein eachpolarity of the first transistor and the second transistor is n-type,and wherein the light emitting element, the first transistor and thesecond transistor are all connected in series.
 2. The light emittingdevice according to claim 1, wherein a ratio of the channel length tothe channel width of the first transistor is equal to or more than
 5. 3.A display device having the light emitting device according to claim 1.4. A digital still camera having the light emitting device according toclaim
 1. 5. A portable information terminal having the light emittingdevice according to claim
 1. 6. A laptop computer having the lightemitting device according to claim
 1. 7. A mobile computer according toclaim 1, wherein the light emitting device is provided.
 8. An imagereproducing device having the light emitting device according toclaim
 1. 9. A goggle type display having the light emitting deviceaccording to claim
 1. 10. A video camera having the light emittingdevice according to claim
 1. 11. A light emitting device comprising: alight emitting element and a first transistor and a second transistoreach for controlling current to be supplied to the light emittingelement, which are formed in a pixel, wherein the first transistor isnormally-on, wherein the second transistor is normally-off, wherein achannel length of the first transistor is longer than a channel widththereof, wherein a channel length of the second transistor is equal toor shorter than a channel width thereof, wherein gate electrodes of thefirst transistor and the second transistor are connected to each other,wherein the first transistor and the second transistor have the samepolarity, and wherein the light emitting element, the first transistorand the second transistor are all connected in series.
 12. The lightemitting device according to claim 11, wherein a ratio of the channellength to the channel width of the first transistor is equal to or morethan
 5. 13. A display device having the light emitting device accordingto claim
 11. 14. A digital still camera having the light emitting deviceaccording to claim
 11. 15. A portable information terminal having thelight emitting device according to claim
 11. 16. A laptop computerhaving the light emitting device according to claim
 11. 17. A mobilecomputer according to claim 11, wherein the light emitting device isprovided.
 18. An image reproducing device having the light emittingdevice according to claim
 11. 19. A goggle type display having the lightemitting device according to claim
 11. 20. A video camera having thelight emitting device according to claim
 11. 21. A light emitting devicecomprising: a light emitting element and a first transistor and a secondtransistor each for controlling current to be supplied to the lightemitting element and a third transistor for controlling input of a videosignal, which are formed in a pixel, wherein the first transistor isnormally-on, wherein the second transistor is normally-off, wherein achannel length of the first transistor is longer than a channel widththereof, wherein a channel length of the second transistor is equal toor shorter than a channel width thereof, wherein gate electrodes of thefirst transistor and the second transistor are connected to each other,wherein the third transistor, the first transistor, and the secondtransistor are connected so that a video signal which is inputted byturning ON the third transistor is given to gate electrodes of the firsttransistor and the second transistor, wherein the first transistor andthe second transistor have the same polarity, and wherein the lightemitting element, the first transistor and the second transistor are allconnected in series.
 22. The light emitting device according to claim21, wherein a ratio of the channel length to the channel width of thefirst transistor is equal to or more than
 5. 23. A display device havingthe light emitting device according to claim
 21. 24. A digital stillcamera having the light emitting device according to claim
 21. 25. Aportable information terminal having the light emitting device accordingto claim
 21. 26. A laptop computer having the light emitting deviceaccording to claim
 21. 27. A mobile computer according to claim 21,wherein the light emitting device is provided.
 28. An image reproducingdevice having the light emitting device according to claim
 21. 29. Agoggle type display having the light emitting device according to claim21.
 30. A video camera having the light emitting device according toclaim
 21. 31. A light emitting device comprising: a light emittingelement and a first transistor and a second transistor each forcontrolling current to be supplied to the light emitting element and athird transistor for controlling input of a video signal and a fourthtransistor for controlling supply of power supply potential, which areformed in a pixel, wherein the first transistor is normally-on, whereinthe second transistor is normally-off, wherein a channel length of thefirst transistor is longer than a channel width thereof, wherein achannel length of the second transistor is equal to or shorter than achannel width thereof, wherein gate electrodes of the first transistorand the second transistor are connected to each other, wherein the thirdtransistor, the first transistor, and the second transistor areconnected so that a video signal which is inputted by turning ON thethird transistor is given to gate electrodes of the first transistor andthe second transistor, wherein the fourth transistor, the firsttransistor, and the second transistor are connected so that the powersupply potential is given to gate electrodes of the first transistor andthe second transistor by turning ON the fourth transistor, wherein thepower supply potential is given to a source of one of the firsttransistor and the second transistor, wherein the first transistor andthe second transistor have the same polarity, and wherein the lightemitting element, the first transistor and the second transistor are allconnected in series.
 32. The light emitting device according to claim31, wherein a ratio of the channel length to the channel width of thefirst transistor is equal to or more than
 5. 33. A display device havingthe light emitting device according to claim
 31. 34. A digital stillcamera having the light emitting device according to claim
 31. 35. Aportable information terminal having the light emitting device accordingto claim
 31. 36. A laptop computer having the light emitting deviceaccording to claim
 31. 37. A mobile computer according to claim 31,wherein the light emitting device is provided.
 38. An image reproducingdevice having the light emitting device according to claim
 31. 39. Agoggle type display having the light emitting device according to claim31.
 40. A video camera having the light emitting device according toclaim
 31. 41. A device substrate comprising: a pixel electrode and afirst transistor and a second transistor each for controlling current tobe supplied to the pixel electrode, which are formed in a pixel, whereina threshold voltage of the first transistor is lower than a threshold ofthe second transistor, wherein a channel length of the first transistoris longer than a channel width thereof, wherein a channel length of thesecond transistor is equal to or shorter than a channel width thereof,wherein gate electrodes of the first transistor and the secondtransistor are connected to each other, wherein each polarity of thefirst transistor and the second transistor is n-type, and wherein thepixel electrode, the first transistor and the second transistor are allconnected in series.
 42. The device substrate according to claim 41,wherein a ratio of the channel length to the channel width of the firsttransistor is equal to or more than
 5. 43. A device substratecomprising: a pixel electrode and a first transistor and a secondtransistor each for controlling current to be supplied to the pixelelectrode, which are formed in a pixel, wherein the first transistor isnormally-on, wherein the second transistor is normally-off, wherein achannel length of the first transistor is longer than a channel widththereof, wherein a channel length of the second transistor is equal toor shorter than a channel width thereof, wherein gate electrodes of thefirst transistor and the second transistor are connected to each other,wherein the first transistor and the second transistor have the samepolarity, and wherein the pixel electrode, the first transistor and thesecond transistor are all connected in series.
 44. The device substrateaccording to claim 43, wherein a ratio of the channel length to thechannel width of the first transistor is equal to or more than
 5. 45. Amethod for driving a light emitting device, comprising the step of:controlling current to be supplied to a light emitting element by afirst transistor and a second transistor, wherein a threshold of thefirst transistor is lower than a threshold of the second transistor,wherein a channel length of the first transistor is longer than achannel width thereof, wherein a channel length of the second transistoris equal to or shorter than a channel width thereof, wherein gateelectrodes of the first transistor and the second transistor areconnected to each other, wherein each polarity of the first transistorand the second transistor is n-type, wherein the light emitting element,the first transistor and the second transistor are all connected inseries, and wherein the first transistor operates in a saturated region,and the second transistor operates in a linear region.
 46. The methodfor driving the light emitting device according to claim 45, wherein aratio of the channel length to the channel width of the first transistoris equal to or more than
 5. 47. A method for driving a light emittingdevice, comprising the step of: controlling current to be supplied to alight emitting element by a first transistor and a second transistor,wherein the first transistor is normally-on, the second transistor isnormally-off; wherein a channel length of the first transistor is longerthan a channel width thereof, wherein a channel length of the secondtransistor is equal to or shorter than a channel width thereof, whereingate electrodes of the first transistor and the second transistor areconnected to each other, wherein the first transistor and the secondtransistor have the same polarity, wherein the light emitting element,the first transistor and the second transistor are all connected inseries, and wherein the first transistor operates in a saturated region,and the second transistor operates in a linear region.
 48. The methodfor driving the light emitting device according to claim 47, wherein aratio of the channel length to the channel width of the first transistoris equal to or more than 5.